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<div>Marijuana plants (<em>Cannabis sativa</em> L.) are cultivated in registered indoor facilities in Nevada for medical use. Powdery mildew is one of the most common diseases in marijuana cultivation facilities, but it is not known what specific fungal species are associated with <em>cannabis</em> plants. In 2017, a powdery mildew infection was observed on the majority of <em>cannabis</em> plants in a cultivation facility, and all allowable fungicides applied provided only limited control of the disease. To better manage the disease, an accurate identification of the causative agent is necessary in order to select a more targeted fungicide. Infected leaves were placed under a dissecting microscope, and mycelium and spores were blotted with a piece of clear tape and mounted on a slide for morphological examination. The fungus produced ellipsoid conidia without the presence of cleistothecia. To further identify the species, 15 µl of TE buffer was applied to the leaf surface where a dense mycelial colony was present. Spores were suspended in the TE buffer droplet by repetitive pipetting, and transferred into a microcentrifuge tube. PCR using primers ITS1/ITS4 was performed after heating the spore solution at 100ºC for 10 minutes. PCR products were purified and sequenced directly. A consensus DNA sequence of 596 bp was obtained and a BLASTn search suggested that this fungus was <em>Golovinomyces ambrosiae</em>. To our knowledge, this is the first report of this species infecting <em>C. sativa</em>.</div>